WASHINGTON - Several months after her identity as a CIA operative was exposed in a newspaper column, Valerie Plame had dinner with five of her classmates from the agency's training academy.

Four had already left the CIA, and they spent the evening catching up on what they'd done during their clandestine careers, as well as the jobs and moves that followed. But even though Plame's "cover" had been cracked wide open, her dinner companions didn't pry for details. Even in that tight circle, no one wanted to spill any more secrets.

"Cover is a mosaic; it's a puzzle," said James Marcinkowski, a former CIA case officer who attended the dinner. "Every piece is important [to protect] because you don't know which pieces the bad guys are missing."

The Plame case brought intense new scrutiny on the White House last week amid disclosures that President Bush's chief political adviser, Karl Rove, is a central figure in the controversy surrounding the "outing" of Plame to the press.

A precious commodity

But the case has also called attention to the precious, concealing commodity the intelligence community calls "cover." The term refers to the amalgam of lies and props, from false names to front companies, that disguise a spy's identity and purpose.

Although often cast in binary terms - an operative is either under cover or not - there are distinct categories of cover that CIA operatives use, and an almost endless list of components. Some cover is tissue-thin and disposable. Other arrangements are so layered and deep that they anticipate hostile probing of every facet of a person's life.

Plame's cover - in which she posed as a private energy consultant while working for a CIA department tracking weapons proliferation - was somewhere in the middle of those extremes.

Minimal damage

Current and former U.S. intelligence officials said it's unlikely that Plame is in any danger as a result of being identified. And an internal review at the CIA concluded that her exposure caused minimal damage, mainly because she had been working at headquarters for years, former officials familiar with the review said.

Still, her clandestine career is over, and the outrage among many current and former case officers lingers because cover is something they go to such great lengths to protect.

"It doesn't matter whether he used her name," Marcinkowski said of the recent disclosures surrounding Rove. "It doesn't matter what her status was. He gave up a piece of the puzzle, and he had no right to do it."

Clandestine service

As many as one-third of the CIA's approximately 20,000 employees are under cover or have worked in that capacity at some point, according to former CIA officials. The agency declined to comment for this article.

The majority of the agency's undercover officers work in the clandestine service - the branch that operates stations around the world, recruiting spies, tracking terrorists and carrying out covert missions designed to influence events or even topple governments.

Plame's identity was revealed in print two years ago by syndicated columnist and conservative commentator Robert Novak. But cover can be compromised in a number of ways.

Inside jobs

The most damaging breaches have often been committed by insiders, such as former CIA officer Aldrich Ames, who was convicted in the mid-1990s of spying for the Russians and revealing dozens of undercover operations and agents' identities to his handlers. In fact, Plame was among those recalled from their overseas assignments at the time, out of concern - never confirmed, former CIA officials said - that she was among those whom Ames had exposed.

More recently, a host of CIA aliases and cover arrangements were exposed in embarrassing fashion by an Italian magistrate. The judge was seeking to prosecute agency operatives for their alleged role in kidnapping a radical Islamic cleric in Milan in 2003 and transferring him to Egypt.